1. Field of the Invention
The present invention relates to a light source device and a projector.
2. Description of Related Art
Conventionally, there has been used a projector that modulates a light beam irradiated from a light source in accordance with image information to project an optical image in an enlarged manner.
A discharge light source device such as a metal halide lamp and a high-pressure mercury lamp has been used as a light source device for such projector (see, for instance, Reference: Japanese Laid-Open Patent Publication No. 08-31382).
The light source device includes; a discharge emitting tube having an emitting portion that generates a light beam by electric discharge between electrodes, and sealing portions provided on both sides of the emitting portion; and a main reflecting mirror having a reflecting surface that irradiates the light beam irradiated from the discharge emitting tube to align the light beam in a predetermined direction.
One of the sealing portions of the discharge emitting tube can be inserted into the main reflecting mirror, the main reflecting mirror having a neck extending along the one of the sealing portions.
The discharge emitting tube is fixed to the main reflecting mirror by positioning light-emitting center of the emitting portion of the discharge emitting tube on a focus position of the main reflecting mirror with the one of the sealing portions of the discharge emitting tube being inserted into the neck of the main reflecting mirror, and by applying an adhesive between the inner circumference of the neck and the outer circumference of the one of the sealing portions.
The discharge emitting tube described above is generally molded by blow-molding as follows.
First, an untreated glass tube is heat-softened on a predetermined position which is to be the emitting portion. Next, a die that defines a shape of the emitting portion is applied to the predetermined portion from the outer circumference of the glass tube, and the predetermined position is inflated by sending air into the glass tube. The air is sent until the outer circumference of the inflated glass tube touches the inner circumferential surface of the die.
In such discharge emitting tube, since the emitting portion is inflated as described above, there exists curved surface connecting the outer circumference of the emitting portion and the outer circumference of the sealing portion in the vicinity of a boundary between the sealing portion and the emitting portion. Inflection points exist in the curved surface connecting the outer circumference of the emitting portion and the outer circumference of the sealing portion. The inflection points is a position changing a refraction direction in which the light irradiated from the emitting center of the emitting portion is reflected when the light is passed through the glass tube and irradiated from the outer circumferential surface of the emitting portion.
Specifically, a light beam irradiated from the emitting center of the emitting portion and irradiated from the outer circumferential surface on the emitting portion side relative to the inflection points (hereinafter referred to as first beam) is likely refracted in a direction orthogonal to an illumination optical axis of the light beam irradiated form the light source device. On the other hand, a light beam irradiated from the emitting center of the emitting portion and irradiated from the outer circumferential surface on the sealing portion side relative to the inflection points (hereinafter referred to as second beam) is likely refracted in a direction toward the illumination optical axis.
Incidentally, the main reflecting mirror described above includes a usable-light reflection area that reflects the light beam irradiated from the emitting portion as a usable light illuminating an object to be illuminated, and two unusable-light reflection areas that cannot reflect the light beam irradiated from the emitting portion as a usable light illuminating the object to be illuminated as described below.
A first unusable-light reflection area is in an opening of the neck. Specifically, in a light beam irradiated from the emitting portion, the light beam traveling toward the opening of the neck is not reflected by the reflecting surface of the main reflecting mirror, which does not illuminate the object to be illuminated.
A second unusable-light reflection area is on a side close to the one of the sealing portions on the reflecting surface of the main reflecting mirror, in other words, in the vicinity of the opening of the neck. More specifically, the second unusable-light reflection area reflects an unusable light that is shielded by the emitting tube and cannot be irradiated on the object to be illuminated even when the light beam irradiated from the emitting portion is reflected by the main reflecting mirror.
When the inflection points on the connecting portion of the emitting portion and the sealing portion are positioned closer to the emitting portion, an area on the sealing portion side relative to the inflection point, where the second beam is irradiated, becomes larger on the outer circumferential surface of the emitting portion from which the light is irradiated. Since the second beam is likely refracted in a direction toward the illumination optical axis, in the light beam irradiated from the emitting portion of the emitting center and traveling toward the usable-light reflection area of the main reflecting mirror under the condition where the refraction on the outer circumferential surface of the emitting portion is not considered, amount of the light beam traveling toward the unusable-light reflection area of the main reflecting mirror might increase. Under such condition, the light utilization efficiency of the light beam irradiated from the emitting portion is lowered. Thus, there has been a demand for a light source device with the positions of the inflection points being defined for enhancing the light utilization efficiency of the light beam irradiated from the emitting portion.